The invention generally relates to systems for treating the surfaces of materials, especially continuously fed, bi-dimensional materials such as webs, sheets, films, papers, textiles, and the like to enhance the surface characteristics of the materials.
Materials, such as paper, polymer webs and various films, are often used in printing, coating and laminating applications. Many polymers, for example, have chemically inert surfaces with low surface energies providing poor bonding properties. To improve bonding, the surface of the material must be treated to raise the surface tension and thereby improve the application of adhesives and inks. There are a number of known techniques for treating the surface of materials to raise surface energy including IR, UV, x-ray and gamma ray irradiation, electron and ion beam bombardment, ozone exposure and flame, chemical, corona and plasma treatments. Furthermore, these techniques are often used to effect surface treatment in large scale operations as part of an assembly or other process line. The material is ordinarily fed at a high rate through a reactor or discharge chamber wherein the discharge and treatment occurs.
In corona treatment, the material is ordinarily fed through the discharge chamber where one surface of the material is bombarded with ions produced by a high voltage alternating electric field. The material can be supported by a backing roll, which can also serve as one electrode for the treater. An active electrode assembly can be supported in the discharge chamber and spaced from the backing roll equidistantly along its length. The active electrode may be either a single integral electrode that extends the entire width of the treater station, or it may be a set of electrode segments, which can be selectively moved into and out of a treatment position to adjust the location, and width of the discharge chamber as disclosed in U.S. Pat. No. 3,409,537. A high voltage AC power supply is connected across the backing roll and active electrodes to produce a corona through which the web passes.
Chemical corona and plasma treaters generally provide a more uniform and controllable surface treatment than corona treaters. In plasma treaters, for example, the discharge chamber is infused with an inert gas that is partially ionized by the energized electrodes. Many plasma treaters require that the pressure of the ionized gas be well below atmospheric pressure. This requires expensive and cumbersome vacuum chambers and pumps to maintain the low pressure. However, more recent treaters can form plasma at atmospheric pressure, see U.S. Pat. No. 5,456,972. In this process, the plasma would form at atmospheric pressure provided an inert gas, typically helium, was used, and a dielectric was applied between the electrodes. Provided that the operating frequency and voltage of the power source are selected to avoid the ambient air from being too readily ionized by the high frequency electric field, a plasma curtain can be formed instead of the lesser quality corona.
Regardless of the type of discharge process, for consistent, high quality treatment it is important to maintain proper conditions within the discharge chamber, which includes maintaining adequate discharge by operating the system at the necessary frequency, maintaining a proper gap distance between the electrodes, and maintaining an adequate flow of process gas throughout the discharge chamber. The latter is particularly important for chemical corona and plasma treatment in which a specific gas or gas chemistry is needed at the discharge of the electrode in order to effect the intended discharge. Maintaining the presence and quality of the gas or gas chemistry in the discharge chamber is particularly difficult for continuous line feed treatment applications because the long web of material passes through the treater at a high speed which causes a boundary layer of air on the web, which is often in a laminar state, to flow into the discharge chamber, thereby displacing and/or diluting the desired gas/gas chemistry. This can also introduce contaminants into the discharge chamber, which can have an adverse effect on the quality and consistency of the treatment, for both corona and plasma treatment. External air cooling of the electrode assembly is also problematic in this regard since the gas/gas chemistry can be inadvertently evacuated from the discharge chamber.
Considerable effort has been expended in this art to safeguard the gas and maintain specific conditions, such as pressure ranges, within the discharge chamber. Many prior art systems, particularly those for use in continuous feed applications and requiring evacuated discharge chambers, employ complex arrangements to seal off the discharge chambers from the ambient environment. For example, U.S. Pat. Nos. 5,314,539; 6,054,018; 6,082,292 and 6,083,355 all disclose systems for maintaining vacuum conditions for continuous feed plasma treaters. Sealed enclosures, vacuum pumps, vacuum lines and remote roller assemblies are used to seal off the discharge chambers. U.S. Pat. No. 5,314,539 uses a number of rollers arranged in series along an arc so that working in stages from outside in a low pressure area can be achieved and maintained between the inner two most rollers where the electrodes are located. Each of the other three patents attempts to improve upon such a multi-staged vacuum arrangement, however, all remain significantly complex to manufacture, assemble, use and maintain. For instance, both U.S. Pat. Nos. 6,054,018 and 6,083,355 require an enclosure housing the electrodes and defining the discharge chamber which has a narrow, remote access opening, through which the web material must be thread, that is sealed by a complex roller arrangement sealing against the web material. Similarly, U.S. Pat. No. 6,082,292 requires a sealed enclosure housing the electrode and defining the discharge chamber as well as two pairs of inside compliant rollers an outside non-compliant rollers to interface with the web material. In all these systems, considerable attention is also given to ensuring that the lateral ends (at each side of the web) are sufficiently sealed in order to achieve and maintain a vacuum.
Furthermore, another significant issue particular to corona treatment, which is typically operated in ambient conditions, is the need to exhaust ozone by-product resulting from the ionization of oxygen-containing air. To prevent odor and poor air quality in the area of the treater, this ozone must be carried away from the treater, possibly decomposed back into oxygen and exhausted from the building. To do this, conventional treaters are integrated with forced air exhaust systems, which often tie into facility air exchange systems to remove the ozone. In addition to the complexity and expense associated with such systems, they can disrupt conditions in the discharge chamber and further exasperate the aforementioned problem.